Lesson 6

The Addition Rule

Lesson Narrative

The mathematical purpose of this lesson is for students to apply the addition rule and to interpret the answer using the model. The work of this lesson connects to previous work because students calculated probabilities using information represented in tables and Venn diagrams. The work of this lesson connects to upcoming work because students will use probability to determine whether or not two events are independent. Students encounter the addition rule which states that given events A and B, \(P(\text{A or B}) = P(\text{A}) + P(\text{B}) - P(\text{A and B})\). When students use the addition rule to get an answer and then interpret the meaning of their answer in a context then they are reasoning abstractly and quantitatively (MP2). When students have to fix or find the error and explain the correct reasoning, they are constructing viable arguments and critiquing the reasoning of others (MP3).

Technology isn't required for this lesson, but there are opportunities for students to choose to use appropriate technology to solve problems. We recommend making technology available.

Learning Goals

Teacher Facing

  • Interpret (orally and in writing) the addition rule in context.
  • Use the addition rule to calculate probabilities.

Student Facing

  • Let’s learn about and use the addition rule.

Learning Targets

Student Facing

  • I can use the addition rule to find probabilities.

CCSS Standards


Glossary Entries

  • addition rule

    The addition rule states that given events A and B, the probability of either A or B is given by \(P(\text{A or B}) = P(\text{A}) + P(\text{B}) - P(\text{A and B})\).

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